The present invention relates to a piezoelectric transducer, or more in particular to an improvement in a piezoelectric resonator used for the stator of an ultrasonic motor and further to an ultrasonic motor utilizing such a resonator.
Conventional ultrasonic motors are roughly divided into standing wave type and travelling wave type, both of which use the rotational torque received by a rotor in pressure contact with the surface of a resonant stator in an elliptical vibrating motion. The stator is made up of a piezoelectric elliptical resonator and is subjected to vibration instead of rotation, with the rotor incessantly continuing to rotate in a direction. In spite of the fact that the stator remains stationary with the rotor rotating, the torque can be transmitted between them by utilizing the static friction among their slipless motion because the vertical component of the elliptical vibration of the stator exerts buoyancy on the rotor and the stator is kept in contact with the rotor only during a half cycle of the vibration and thereby the torsional component of the vibration exerts a uni-directional rotational torque, on the rotor while the stator comes away from the rotor to return in the reverse direction during the remaining half cycle. When this principle is satisfied, the abrasion can be avoided.
Satisfaction of this principle, however, requires a very complicated mechanism for the travelling wave type, and both production and operation thereof is required to be based on a highly sophisticated technology. The basic reason for this lies in the disadvantage that the piezoelectric elliptical resonator does not rotate in real terms. Another disadvantage is that a piezoelectric elliptical resonator of standing wave type cannot be constructed in thin form and therefore the motor becomes thick, thereby making it impossible to reduce the thickness of the motor.
Further, the torque and output of an ultrasonic motor depends on the momentum associated with the vibration of the stator. In the conventional ultrasonic motors, only a fraction of the total mass is involved in the effective motion of the stator. This is by reason of the fact that the vibration generated in the stator has an amplitude distribution which is generally about 1/.pi. or 1/4 the total mass. In this way, the motion associated with the stator vibration cannot be utilized in full, resulting in torque and output drop to 1/.pi. or 1/4.